If a kaafir has a cheese business where he slaughters animals (improperly) just for their rennet, then is it permissible to get cheese from him?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

In principle, all kinds of food, other than the slaughtered animals, are lawful and permissible to eat as long as the person does not know for certain that there is something forbidden in their ingredients.

If a person doubts about something, then out of piety, it is better and safer to leave it and stay away from it even if the additives to the cheese were impure but they were treated until they completely transformed (into another matter) and became lawful before adding them to the foodstuffs.

Many scholars stated that they are lawful, and this is the view of the Hanafi, the Thaahiriyyah [those who interpret the Islamic texts literally], and the Maaliki School in their famous view.

However, if these additives have not been processed, or were processed but they were not transformed into another matter, then they remain according to their origin, which is that they are impure and forbidden to use, because by mixing that impure matter with it they become impure; in which case, one should refrain from trading it.

For more benefit, please refer to Fataawa 370123, 347143 and 81774.

Allah knows best.